The present invention relates to a method and plant for treating the process sludges resulting from the manufacture and machining of stone materials, ceramic materials and agglomerated stone materials, in particular in the case of the latter, those obtained using the so-called Bretonstone technology, to which for the sake of easier illustration reference will be made in the remainder of this description, without being regarded as limited thereto.
The Bretonstone technology in question essentially envisages the preparation of a mix comprising a granulated stone material or the like, with a suitably selected particle size, and a binder consisting of an organic resin which can be hardened, the mix preferably having, added thereto, a very fine inorganic agent, called filler, preferably formed by powdery stone material.
The mix is deposited in the form of a thin layer on a temporary support or used for filling a suitable mould (in the case where a block is to be formed).
Then the mix is subject to a vacuum compression operation and at the same time a vibratory movement at a predetermined frequency.
The rough-formed article thus obtained is transferred so as to undergo a step involving hardening of the binding resin by means of the action of heat and/or a catalyst.
For further details concerning this technology refer, for example, to the
Following manufacture of the slabs, it is then required to perform finishing operations such as sizing and polishing using special abrasive tools operating under a continuous stream of water.
The slabs of stone or ceramic materials are subject to similar operations.
The stream of water is used to cool the tools of the sizing and polishing machines and at the same time eliminate the dust generated by the machining operation. These abrasive tools may be composed of diamond-coated granules which are embedded in a metallic or resinous base.
These operations generate, in terms of waste, a powder consisting of stone material (for example marble, granite, quartz) or ceramic material and any hardened resinous binder as well as any additives, this powder remaining suspended in the abovementioned water stream.
In the case of blocks these undergo firstly a sawing operation which is performed using diamond-coated blades which also operate under a stream of water, with the formation of a similar powdery suspension such as that mentioned above.
This waste water is normally subject to a clarification treatment and remains within the circuit in order to supply the circuits for cooling the sizing, polishing and cutting tools. For this purpose, the machining water, which contains said dispersion powders and to which various substances (e.g. flocculating and anti-foaming agents) are normally added, is treated in separation apparatus or settling tanks so that the powder separates while the water is recycled in order to supply the circuits for cooling the sizing, polishing and cutting tools.
The sludges, accumulated inside the separation or settling devices, are fairly fluid and still have a high water content.
For this reason, during an initial phase of the cycle, the sludges are conveyed into concentration apparatus, such as centrifuges or more often filter presses so that as much water as possible can be extracted.
The water thus extracted is also recycled and used for machining of the slabs of stone, ceramic or agglomerated stone material.
The concentrated sludges, which have a water content varying between 15% and 30% depending on the type of concentration machine, are transferred to refuse collection centres (waste tips) with the consequent problems and costs.
Essentially the machining sludges comprise:
The international patent application
The aim of the treatment is to recover the water contained in the sludges and the heat used for drying thereof.
The Italian patent application
The plant comprises, downstream of the filter press, a drier for removal of the water by means of evaporation, an incinerator for eliminating by means of calcination the organic substances contained in the dried sludges, with a post-combustion facility.
The plant also comprises:
The particle size of powdery stone material thus obtained depends, not only on the nature of the material itself, but also the material grain of the tools used during the cutting and sizing/polishing operations.
However, even using this method, it is not possible to eliminate entirely the need to use a refuse collection centre, on account of the powders produced.
The object of the present invention is therefore to improve further the abovementioned technology and in particular to provide a method and a plant which allows an efficient use of the machining sludges from a manufacturing and cost-related point of view, resulting in materials useful for other uses.
A second task of the present invention is to provide a method and a plant which allow the products resulting from the treatment of sludges, should they not be reutilized, to be disposed of in an ordinary waste disposal tip for aggregate material.
The object and tasks of the present invention are achieved with a method and a plant as defined in the claims.
The further advantages and characteristic features of the present invention will become clear from the detailed description which follows of a number of examples of embodiment, provided by way of a non-limiting example, with reference to the accompanying drawing which illustrates in schematic form the sequence of treatment steps and therefore also the corresponding plant diagram::
The steps of the method according to the invention also comprise:
In Figure 1 the arrow F1 denotes rough-formed slabs at the inlet of plant, while the arrow F2 denotes finished slabs leaving the plant. If the slabs are obtained by means of sawing of blocks, known sawing frames (not shown in the figures) for sawing the blocks into slabs are provided upstream of the abovementioned machines.
In the diagram shown in the figure, the reference number 10 denotes a sizing machine for sizing the rough-formed slabs at the inlet, the reference number 12 denotes a polishing machine, and the reference number 14 denotes a cutting machine.
The reference number 16 denotes a header for supplying the cooling water to the abovementioned machines via the branch lines 16A, 16B and 16C.
The machining water, which has the composition described further above and is supplied by the machines 10, 12 and 14, is conveyed into a receiving header 18 via branch lines 18A, 18B and 18C.
A pipe 20 for supplying any suitable settling additives, for example flocculating and anti-foam agents, which are drawn from a tank 22, is connected to the header 18.
The header 18 leads into a settling tank 24 from where the supernatant water is drawn off by means of the pipe 26 and is recycled via the pipe 28 to the header 16 situated in a position upstream of the machines 10, 12 and 14.
If necessary, replenishing water is supplied via a pipe 30.
In this way the water extracted from the sludges can be used along the same slab processing line.
The sludges discharged from the settling tank 24 are transferred by means of a pipe 32 to a concentrator 34, for example a filter press of the conventional and well-known type, having the function of further extracting water form the sludges so as that the latter are concentrated.
The water recovered in this way is conveyed via a pipe 35 to the pipe 26.
The concentrated sludges discharged by the concentrator 34 are collected in a standing tank 36 from where, via a pipe 38, they are conveyed into a pre-drier 40. The function of the pre-drier 40, which is of the conventional and well-known type, is to reduce the moisture in the sludges produced by the concentrator 34 to a value less than about 16%.
A pipe 42 connects the pre-drier 40 to a granulator 44 for granulation of the sludges.
The granulator 44 consists essentially of a mixing machine into which the sludge to be treated is introduced via the pipe 42, together with an inorganic binder, via a metering device 46 connected to the granulator by means of the pipe 48.
The inorganic binder is an aqueous solution, for example a silicate of an alkaline metal such as sodium or potassium.
The reason why a pre-drier is used for preparing the sludges for the granulation step is therefore clear.
In fact, since the aforementioned liquid binder results in an increase in the water content of the sludges, introducing the sludges into the granulator 44 without using a pre-drier 40 would result in the mixture being too fluid for the granulation step.
During the mixing step described above it is possible to add clayey material or zinc oxide, these having the function of making the binder completely insoluble after the subsequent baking step.
The granules thus obtained, which have a size of a few millimetres, are transferred to a drier 50 inside which evaporation of the water contained in the granulate is performed, followed by transfer of the latter into a baking oven 52 operating at a temperature sufficient to ensure insolubilization of the binder and elimination by means of combustion of all the organic substances contained in the sludges. The temperature of the baking oven 52 may range between 400°C and 650°C and the granules remain inside it for a duration such as to ensure the aforementioned complete insolubilization and combustion of the organic substances (for example in the region of at least 10 minutes).
Preferably, the temperature of the baking oven 52 is between 550°C and 600°C.
Both the drier 50 and the baking over 52 are well-known apparatus.
It is also envisaged that the drier 50 and the baking oven 52 may form a single incineration apparatus which performs both functions.
The water removed by means of evaporation during the drying step may be condensed inside the condenser 54 and recycled together with the water recycled from the settling tank via the pipe 56.
The fumes from the baking oven 52 are conveyed to a post-combustion unit 58, operating normally at temperatures suitable for ensuring that the fumes do not contain toxic substances (in particular dioxin), said unit therefore being made to operate at temperatures of between 650°C and 850°C.
The fumes from the post-combustion unit 58 are then transferred to a filter 60 which retains any powder contained in the said fumes which is conveyed back via a pipe 62 to the granulator 44.
The granulate output from the baking oven 52 (arrow F3) has a low apparent specific weight of about 1.1-1.3 Kg/dm3 and is devoid of organic substances.
In an alternative embodiment the granulation and drying operation may be performed in a single apparatus, such as the so-called "spray-dry" apparatus.
Said apparatus is provided with means for spraying the mixture, which comprises sludge and binder, together with any added clayey material or zinc oxide, thus generating droplets of mixture, within a high-temperature air stream which removes most of the moisture present, forming the granules which in this case have a size of normally smaller than 1 mm.
In this embodiment of the invention it is no longer necessary to use a pre-drier since spraying requires a mixture of sludge and binder which is fairly fluid.
The advantage achieved by means of the present invention is that of obtaining a finished product consisting of granules which are devoid of organic components, suitable for many uses in the building industry, for example being able to be used in the production of lightweight concretes or the manufacture of filling materials. The granules may also be used for the manufacture of agglomerated stone products.
A further advantage of the present invention is that the granulate obtained, in the case of an excessive amount being produced, may be disposed of extremely easily in a waste tip for aggregate material since it is not in powder form and does not contain organic materials.
A process devoid of machining waste is thus provided, therefore eliminating the problem and the cost associated with the disposal of waste containing organic components in special waste tips.
The process for manufacturing the agglomerated stone material, in particular Bretonstone slabs and blocks, is therefore a process which respects to a greater degree the environment, in particular if the resin used is of vegetable origin as described in the patent application
It is very important to point out that the resin contained in the agglomerated stone sludges, during the process, acts as a fuel, so that the energy balance is practically equal to zero. In other words, the heat produced by the combustion of the resin contained in the sludges is nearly sufficient to ensure evaporation of all the water and heating of the dry residue up to about 450-600° C.
The gases generated during calcination and subsequent post-combustion may also pass through a heat exchanger so as to recover the heat contained in them, and the latter used to pre-heat the air required for the drying step. The energy requirement for this treatment step is therefore substantially limited.
The invention has been described with reference to preferred embodiments, but it is understood that conceptually equivalent modifications and variations are possible and may be applied.
For example, in order to form granules, the concentrated sludge to which inorganic binder has been added may be converted into granules by means of a known pelletization process.
This list of references cited by the applicant is for the reader's convenience only. It does not form part of the European patent document. Even though great care has been taken in compiling the references, errors or omissions cannot be excluded and the EPO disclaims all liability in this regard.